Retroreflective articles and devices having viscoelastic lightguide

ABSTRACT

Disclosed herein is an optical device having a light source, a viscoelastic lightguide and a retroreflective film suitable for retroreflecting light. Light from the light source enters the viscoelastic lightguide and is transported within the lightguide by total internal reflection. The optical device may have a “front lit” configuration such that light being transported within the lightguide is extracted and retroreflected by the film toward a viewer. The optical device may have a “back lit” configuration such that light being transported within the lightguide is extracted and transmitted through the film toward a viewer. The retroreflective film may comprise beaded retroreflective sheeting such as that used in traffic signs and markings.

FIELD

This disclosure relates to optical articles and devices, particularlythose that are retroreflective. The optical articles and devices includelightguides made with viscoelastic materials.

BACKGROUND

Retroreflective films are characterized by the ability to reflectincident light back toward an originating light source. Microsphereretroreflective sheeting, sometimes referred to as “beaded”retroreflective sheeting, typically comprises a monolayer of transparentmicrospheres supported by a binder. Light incident upon a front surfaceof the sheeting is retroreflected by the transparent microspheres anddirected back toward the light source. A reflective material disposedbehind the microspheres may be present to facilitate retroreflection.Beaded retroreflective sheeting may include a composite image that canprovide a floating image that appears to be suspended above or below thesheeting. Beaded retroreflective sheeting may be used in traffic safetyapplications, such as for license plates, road signs, barricades,pavement markers and marking tape, as well as for personal safetyapplications including tape for clothing, headgear, vehicles and thelike. Beaded retroreflective sheeting may be used to provide signage ingraphic arts applications.

Beaded retroreflective sheeting is known for being able to reflect alarge portion of incident light back towards an originating lightsource. Without a light source, however, beaded retroreflective sheetingcan be difficult to see under some conditions.

Lightguides are used to facilitate distribution of light from a lightsource over an area much larger than the light source. Lightguidescomprise optically transmissive materials and may have different formssuch as slab, wedge, and pseudo-wedge forms. Most lightguides aredesigned to accept light at an edge surface and allow this light topropagate by total internal reflection between a back surface and anoutput surface, toward an opposing edge surface from which the lightenters. Light is emitted uniformly from the output surface usingextracting features that are positioned in various types of patterns onthe output surface.

SUMMARY

Disclosed herein is an optical device having a light source, aviscoelastic lightguide and a retroreflective film suitable forretroreflecting light. Light from the light source enters theviscoelastic lightguide and is transported within the lightguide bytotal internal reflection. The optical device may have a “front lit”configuration such that light being transported within the lightguide isextracted and retroreflected by the film toward a viewer. In some cases,the optical device may have a “back lit” configuration such that lightbeing transported within the lightguide is extracted and transmittedthrough the film toward a viewer. The retroreflective film may comprisebeaded retroreflective sheeting such as that used in traffic signs andmarkings.

The optical device may be used as, for example, as a sign or marking, alicense plate assembly, a tail light assembly for vehicles, a securitylaminate for protection of documents against tampering, or anillumination device.

These and other aspects of the invention are described in the detaileddescription below. In no event should the above summary be construed asa limitation on the claimed subject matter which is defined solely bythe claims as set forth herein.

BRIEF DESCRIPTIONS OF DRAWINGS

Advantages and features of the invention may be more completelyunderstood by consideration of the following figures in connection withthe detailed description provided below. The figures are schematicdrawings of various devices and articles and are not necessarily drawnto scale.

FIGS. 1-18 show schematic cross sections of exemplary optical deviceshaving front lit configurations.

FIG. 19 shows a schematic cross section of an exemplary device having afront lit configuration and that provides a floating image.

FIG. 20 shows a schematic cross section of an exemplary device having aback lit configuration and that provides a floating image.

FIG. 21 shows a schematic cross section of an exemplary device havingfront and back configurations and that provides a floating image.

DETAILED DESCRIPTION

The optical device disclosed herein includes a light source that emitslight, a viscoelastic layer for managing the light and a retroreflectivefilm for redirecting the light back in the general direction of thelight source. Optical article refers to the corresponding optical devicewithout the light source.

The optical device may provide one or more advantages. For example, theviscoelastic lightguide is generally soft and compliant such that thelight source may be easily coupled to the lightguide so that light canenter the lightguide. In some embodiments, the viscoelastic lightguidecomprises a PSA which is generally tacky at room temperature. The lightsource may then be coupled to the viscoelastic lightguide such that itis adhered to the lightguide. This may facilitate assembly of theoptical device itself or constructions in which the device is used.

Light is typically extracted from the viscoelastic lightguide at one ormore desired locations or areas of the lightguide. In some embodiments,an extractor layer may be used to extract light from the viscoelasticlightguide. Again, due to the soft and compliant properties of theviscoelastic lightguide, the extractor layer may be easily coupled tothe lightguide so that light can enter the layer. If the viscoelasticlightguide comprises a PSA, the extractor layer can be directly adheredto the lightguide without the need for additional materials to bond thetwo together. Light from the extractor layer can then be retroreflectedby the retroreflective film such that the optical article is lit up bylight originating from the light source.

In some embodiments, the retroreflective film may be used to extractlight from the viscoelastic lightguide. Again, due to the soft andcompliant properties of the viscoelastic lightguide, the retroreflectivefilm may be easily coupled to the lightguide so that light can enter theretroreflective film. If the viscoelastic lightguide comprises a PSA,the retroreflective film can be directly adhered to the lightguidewithout the need for additional materials to bond the two together.Light from the viscoelastic lightguide can then be retroreflected by theretroreflective film such that the optical article is lit up by lightoriginating from the light source.

The optical device may be used to provide light anywhere it is desired.The optical device may be designed for interior and/or exterior use. Theoptical device may be designed for household, commercial and/orindustrial use. The optical device may be used and/or provided in aconstruction so that it is portable, i.e., it is a portable source oflight. Lighted cards, tapes, signs, labels, stickers, cut-outs, etc. areexamples of portable constructions that may be made using the opticaldevice. The optical device may also be used and/or provided in a morestationary construction such as in a license plate assembly or as partof a lighting assembly used to provide lighting on the exterior of avehicle, e.g., for tail lights, replacing tail light cavities and theirlighting assemblies and which are very space consuming. The opticaldevice may also be used to provide “light on demand”, e.g., the lightsource may be activated selectively when certain conditions are met.

The optical device may also be very adaptable, even by a user, so thatit can be used in different lighting forms and constructions. Forexample, optical articles may be provided in roll or sheet form that canbe cut into various shapes and sizes. The light source may beinterchangeable with the optical article, for example, if the lightsource should become unusable or if a different color of light isdesired. Further, if used in a sign construction, graphics can beinterchanged, for example, if one would like to update an advertisement.The optical device can be used with a variety of retroreflective films,and the films can be either front lit or back lit relative to theposition of a viewer.

The optical device may provide many more advantages. The optical devicecan be used to provide light that is bright, diffuse, uniform and/orconcentrated over particular areas. The optical device may provideadvantages by being thin, flexible and/or lightweight. The viscoelasticlightguide may be tiled to light large areas of retroreflective filmwhich may be made easier if the lightguides can be stuck together. Dueto its viscoelastic properties, the viscoelastic lightguide may alsodampen stresses experienced by the optical device or construction inwhich the device is used. The viscoelastic lightguide, when disposed ona substrate, may be removable and/or repositionable over time. Theoptical device may also provide advantages related to cost, because itcan be made from commercially available light sources, viscoelasticmaterials and retroreflective films. Additional advantages are describedbelow.

The behavior of light with respect to the optical devices and articlesdisclosed herein can be described using principles of geometric optics.These principles are well known and are not presented here; a moredetailed description can be found in the Sherman et al. references citedabove. In general, one may apply the law of refraction and the principleof total internal reflection in conjunction with ray tracing techniquesto determine theoretically how varying three dimensional structure,material composition, layer construction, angular distribution of light,etc. can affect the behavior of light for the optical devices andarticles disclosed herein.

FIG. 1 shows a schematic cross section of exemplary optical device 100.This embodiment is an example of a front lit configuration in whichviscoelastic lightguide 110 is on top of retroreflective film 140 orcloser than the retroreflective film to the viewer as indicated by eye120. Light source 105 is positioned relative to viscoelastic lightguide110 such that light emitted by the light source enters viscoelasticlightguide 110 and is transported within the lightguide by totalinternal reflection. Light emitted by the light source is represented byrays 106 which enter viscoelastic lightguide 110 through an inputsurface (not shown) adapted to receive light from the light source.Light within the viscoelastic lightguide is represented by single ray130 which is transported by total internal reflection. At least aportion of the viscoelastic lightguide has optically smooth surface 111and/or 112.

Light emitted by the light source enters the viscoelastic lightguide andis transported within the lightguide by total internal reflection. Ingeneral, total internal reflection occurs when light having a particularangular component or distribution is incident upon an interface at oneor more angles greater than the critical angle θ_(c). An opticallysmooth surface, as used herein, means that the surface is smooth enoughsuch that light incident upon the surface is not affected undesirably bythe surface, e.g., the surface is free of defects having at least onedimension larger than the wavelength of the incident light. Theoptically smooth surface allows at least some of the light entering theviscoelastic lightguide to be reflected at the surface such that thislight continues to propagate within the layer according to the principleof total internal reflection. For reflection of light incident on anoptically smooth surface, the observed reflection angle is within about10° of the calculated reflection angle. Total internal reflection occursif a predetermined amount, or at least within about 10% of apredetermined amount, of light does not escape the viscoelasticlightguide unless it is intentionally extracted from the lightguide.

The viscoelastic lightguide may have opposing major surfaces that aresubstantially unstructured as shown in FIG. 1 for surfaces 111 and 112.These major surfaces may also be structured with a plurality offeatures, or one major surface may be substantially unstructured and theother structured with a plurality of features.

A structured surface of the viscoelastic lightguide comprises aplurality of features which may include protrusions and/or depressionshaving lenticular, prismatic, ellipsoidal, conical, parabolic,pyramidal, square, or rectangular shapes, or a combination thereof.Features comprising lenses are particularly useful for directing lightto a preferred angular distribution. Features comprising linear prismsor elongated prisms are also particularly useful. Other exemplaryfeatures comprise protrusions and/or depressions having elongated,irregular, variably sloped lenticular, or random columnar shapes, or acombination thereof. Hybrids of any combination of shapes may be used,for example, elongated parabolic, pyramidal prismatic, rectangular-basedprismatic, and rounded-tip prismatic shapes. The features may compriserandom combinations of shapes.

Sizes of the features may be described by their overall shapes in threedimensions. In some embodiments, each feature may have a dimension offrom about 1 to about 100 um, for example, from about 5 to about 70 um.The features of a surface may have all the same shape, but the sizes ofthe shapes may vary in at least one dimension. The features of a surfacemay have different shapes, and the sizes of these features may or maynot vary in any given dimension.

Surface structures of the features may also be varied. Surface structureof a feature generally refers to the sub-structure of the feature.Exemplary surface structures include optically smooth surfaces,irregular surfaces, patterned surfaces, or a combination thereof. For agiven surface having a plurality of features, all or some of thefeatures may have the same surface structure, or they may all bedifferent. The surface structure of a feature may vary over portions ofthe feature. An optically smooth surface of a feature may form part ofthe optically smooth surface of the viscoelastic lightguide. Theoptically smooth surfaces of the feature and the viscoelastic lightguidemay be continuous or discontinuous with each other. If a plurality offeatures is used, the surfaces of some features may be completelyoptically smooth or some may be partially optically smooth.

The number of features, if used, for a given structured surface is atleast one. A plurality of features, meaning at least two, may also beused. In general, any number of features may be included, e.g., 0, 1, 2,3, 4 or 5 features; greater than 1, greater than 10, greater than 20,greater than 30, greater than 40, greater than 50, greater than 100,greater than 200, greater than 500, greater than 1000, or greater than2000 features; or from about 1 to about 10, from about 1 to about 20,from about 1 to about 30, from about 1 to about 40, from about 1 toabout 50, from about 1 to about 100, from about 1 to about 200, fromabout 1 to about 500, from about 1 to about 1000, or from about 1 toabout 2000 features.

The features may be randomly arranged, arranged in some type of regularpattern, or both. The distance between features may also vary. Thefeatures may be discreet or they may overlap. The features may bearranged in close proximity to one another, in substantial contact witheach other, immediately adjacent each other, or some combinationthereof. A useful distance between features is up to about 10 um, orfrom about 0.05 um to about 10 um. The features may be offset withrespect to one another, angularly as well as transversely. The arealdensity of the features may change over the length, width, or both.

The features may be used to control the amount and/or direction of lightthat is extracted from the retroreflective film. The features may bearranged to obtain a desired optical effect. The features may bearranged to provide an image, extract light uniformly or as a gradientfrom the viscoelastic lightguide, hide discrete light sources, or reduceMoiré. This can be carried out generally by varying the shape, size,surface structure, and/or orientation of the features. If a plurality offeatures is used, then the number and/or arrangement of the features maybe varied, as well as the orientation of the features relative to eachother.

The shape of a feature may change the angular component of light whichcan increase or decrease the amount of light that is extracted from theviscoelastic layer. This may be the case if light propagates by totalinternal reflection within the viscoelastic lightguide and strikes asurface of a feature at an angle less than, equal to, or greater thanthe critical angle for the viscoelastic lightguide and an adjacentsubstrate which may or may not be the retroreflective film. The amountof light that is extracted from the viscoelastic lightguide may increaseor decrease accordingly. The size of a feature may be changed such thatmore or less light can reflect off a surface of the feature, thusincreasing or decreasing the amount of light that is extracted from theviscoelastic layer. The surface structure of a feature may be used tocontrol the distribution of light that is extracted from theviscoelastic layer. Light having a particular angular distribution maystrike a feature and be extracted uniformly and/or randomly. Light mayalso be extracted uniformly and in a pattern, or randomly and in apattern.

The viscoelastic lightguide is generally in contact with at least onemedium. The medium may comprise air or a substrate, and substrates maybe the retroreflective film, polymeric film, metal, glass, and/orfabric. Particular substrates are described below for a variety ofexemplary constructions. For the purpose of convenience, a viscoelasticlightguide in contact with a substrate is described below, but thissubstrate may comprise any type of medium including a polymeric film,the retroreflective film or air.

Given a particular substrate in contact with the viscoelasticlightguide, the amount of light extracted from the lightguide and by thesubstrate may be from about 10 to about 50%, from about 20 to about 50%,from about 30 to about 50%, from about 50 to about 70%, from about 50 toabout 80%, or from about 10 to about 90% relative to the total amount oflight that enters the lightguide.

The transmittance angle for light extracted from the viscoelasticlightguide by the retroreflective film or substrate may be from greaterthan about 5° to less than about 95°, greater than about 5° to less thanabout 60°, or greater than about 5° to less than about 30°.

The viscoelastic lightguide may have a refractive index greater thanthat of the retroreflective film or the substrate. The refractive indexof the viscoelastic lightguide may be greater than about 0.002, greaterthan about 0.005, greater than about 0.01, greater than about 0.02,greater than about 0.03, greater than about 0.04, greater than about0.05, greater than about 0.1, greater than about 0.2, greater than about0.3, greater than about 0.4, or greater than about 0.5, as compared tothe refractive index of the retroreflective film or substrate.

The viscoelastic lightguide may have a refractive index less than thatof the retroreflective film or substrate. The refractive index of theviscoelastic lightguide may be less than about 0.002, less than about0.005, less than about 0.01, less than about 0.02, less than about 0.03,less than about 0.04, less than about 0.05, less than about 0.1, lessthan about 0.2, less than about 0.3, less than about 0.4, or less thanabout 0.5, as compared to the refractive index of the retroreflectivefilm or substrate.

The viscoelastic lightguide and the retroreflective film or substratemay have the same or nearly the same refractive index such that lightcan be extracted into the retroreflective film or substrate with littleor no change to the light. The refractive index difference of theviscoelastic lightguide and the retroreflective film or substrate may befrom about 0.001 to less than about 0.002.

The refractive index difference of the viscoelastic lightguide and theretroreflective film or substrate may be from about 0.002 to about 0.5,from about 0.005 to about 0.5, from about 0.01 to about 0.5, from about0.02 to about 0.5, from about 0.03 to about 0.5, from about 0.04 toabout 0.5, from about 0.05 to about 0.5, from about 0.1 to about 0.5,from about 0.2 to about 0.5, from about 0.3 to about 0.5, or from about0.4 to about 0.5.

The viscoelastic lightguide may have any bulk three-dimensional shape asis needed for a given application. The viscoelastic lightguide may be inthe form of a square or rectangular layer, sheet, film, etc. Theviscoelastic lightguide may be cut or divided into shapes as describedbelow.

The thickness of the viscoelastic lightguide is not particularly limitedas long as it can function as desired. The thickness of the viscoelasticlightguide may be selected based on or in conjunction with the lightsource. For example, design parameters may limit or even require that aparticular light source(s) be used, and there may be a minimum amount,or range of amounts, of light that is required to enter the viscoelasticlightguide. Thus, the thickness of the viscoelastic lightguide may beselected so that the required amount of light from a given light sourcecan enter the lightguide. A maximum thickness of the viscoelasticlightguide may be required for use in optical devices designed to beparticularly thin. Exemplary thicknesses for the viscoelastic lightguiderange from about 0.4 mil to about 1000 mil, from about 1 mil to about300 mil, from about 1 mil to about 60 mil, or from about 0.5 mil toabout 30 mil.

The amount and direction of light extracted from the viscoelasticlightguide may be controlled, at the very least, by the shape, size,number, arrangement, etc. of the features, the refractive indices of theviscoelastic lightguide and any medium with which the lightguide is incontact, the shape and size of the viscoelastic lightguide, and theangular distribution of light that is allowed to enter the viscoelasticlightguide. These variables may be selected such that from about 10 toabout 50%, from about 20 to about 50%, from about 30 to about 50%, fromabout 50 to about 70%, from about 50 to about 80%, or from about 10 toabout 90% of light is extracted from the viscoelastic lightguiderelative to the total amount of light that enters the lightguide.

The viscoelastic lightguide comprises one or more viscoelasticmaterials. In general, viscoelastic materials exhibit both elastic andviscous behavior when undergoing deformation. Elastic characteristicsrefer to the ability of a material to return to its original shape aftera transient load is removed. One measure of elasticity for a material isreferred to as the tensile set value which is a function of theelongation remaining after the material has been stretched andsubsequently allowed to recover (destretch) under the same conditions bywhich it was stretched. If a material has a tensile set value of 0%,then it has returned to its original length upon relaxation, whereas ifthe tensile set value is 100%, then the material is twice its originallength upon relaxation. Tensile set values may be measured using ASTMD412. Useful viscoelastic materials may have tensile set values ofgreater than about 10%, greater than about 30%, or greater than about50%; or from about 5 to about 70%, from about 10 to about 70%, fromabout 30 to about 70%, or from about 10 to about 60%.

Viscous materials that are Newtonian liquids have viscouscharacteristics that obey Newton's law which states that stressincreases linearly with shear gradient. A liquid does not recover itsshape as the shear gradient is removed. Viscous characteristics ofuseful viscoelastic materials include flowability of the material underreasonable temperatures such that the material does not decompose.

The viscoelastic lightguide may have properties that facilitatesufficient contact or wetting with at least a portion of theretroreflective film or substrate such that the lightguide and the filmor substrate are optically coupled. Light can then be extracted from theviscoelastic lightguide by the film or substrate. The viscoelasticlightguide is generally soft, compliant and flexible. Thus, theviscoelastic lightguide may have an elastic modulus (or storage modulusG′) such that sufficient contact can be obtained, and a viscous modulus(or loss modulus G″) such that the layer doesn't flow undesirably, and adamping coefficient (G″/G′, tan D) for the relative degree of damping ofthe layer.

Useful viscoelastic materials may have a storage modulus, G′, of lessthan about 300,000 Pa, measured at 10 rad/sec and a temperature of fromabout 20 to about 22° C. Useful viscoelastic materials may have astorage modulus, G′, of from about 30 to about 300,000 Pa, measured at10 rad/sec and a temperature of from about 20 to about 22° C. Usefulviscoelastic materials may have a storage modulus, G′, of from about 30to about 150,000 Pa, measured at 10 rad/sec and a temperature of fromabout 20 to about 22° C. Useful viscoelastic materials may have astorage modulus, G′, of from about 30 to about 30,000 Pa, measured at 10rad/sec and a temperature of from about 20 to about 22° C. Usefulviscoelastic materials may have a storage modulus, G′, of from about 30to about 150,000 Pa, measured at 10 rad/sec and a temperature of fromabout 20 to about 22° C., and a loss tangent (tan d) of from about 0.4to about 3. Viscoelastic properties of materials can be measured usingDynamic Mechanical Analysis according to, for example, ASTM D4065,D4440, and D5279.

In some embodiments, the viscoelastic lightguide comprises a PSA layeras described in the Dalquist criterion line (as described in Handbook ofPressure Sensitive Adhesive Technology, Second Ed., D. Satas, ed., VanNostrand Reinhold, N.Y., 1989.)

The viscoelastic lightguide may have a particular peel force or at leastexhibit a peel force within a particular range. For example, theviscoelastic lightguide may have a 90° peel force of from about 50 toabout 3000 On, from about 300 to about 3000 On, or from about 500 toabout 3000 On. Peel force may be measured using a peel tester fromIMASS.

In some embodiments, the viscoelastic lightguide comprises an opticallyclear lightguide having high light transmittance of from about 80 toabout 100%, from about 90 to about 100%, from about 95 to about 100%, orfrom about 98 to about 100% over at least a portion of the visible lightspectrum (about 400 to about 700 nm). In some embodiments, theviscoelastic lightguide has a haze value of less than about 5%, lessthan about 3%, or less than about 1%. In some embodiments, theviscoelastic lightguide has a haze value of from about 0.01 to less thanabout 5%, from about 0.01 to less than about 3%, or from about 0.01 toless than about 1%. Haze values in transmission can be determined usinga haze meter according to ASTM D1003.

In some embodiments, the viscoelastic lightguide comprises an opticallyclear lightguide having high light transmittance and a low haze value.High light transmittance may be from about 90 to about 100%, from about95 to about 100%, or from about 99 to about 100% over at least a portionof the visible light spectrum (about 400 to about 700 nm), and hazevalues may be from about 0.01 to less than about 5%, from about 0.01 toless than about 3%, or from about 0.01 to less than about 1%. Theviscoelastic lightguide may also have a light transmittance of fromabout 50 to about 100%.

In some embodiments, the viscoelastic lightguide is hazy and diffuseslight, particularly visible light. A hazy viscoelastic lightguide mayhave a haze value of greater than about 5%, greater than about 20%, orgreater than about 50%. A hazy viscoelastic lightguide may have a hazevalue of from about 5 to about 90%, from about 5 to about 50%, or fromabout 20 to about 50%.

In some embodiments, the viscoelastic lightguide may be translucent inthat it reflects and transmits light.

The viscoelastic lightguide may have a refractive index in the range offrom about 1.3 to about 2.6, 1.4 to about 1.7, or from about 1.5 toabout 1.7. The particular refractive index or range of refractiveindices selected for the viscoelastic lightguide may depend on theoverall design of the optical device and the particular application inwhich the device may be used.

The viscoelastic lightguide generally comprises at least one polymer.The viscoelastic lightguide may comprise at least one PSA. PSAs areuseful for adhering together adherends and exhibit properties such as:(1) aggressive and permanent tack, (2) adherence with no more thanfinger pressure, (3) sufficient ability to hold onto an adherend, and(4) sufficient cohesive strength to be cleanly removable from theadherend. Materials that have been found to function well as pressuresensitive adhesives are polymers designed and formulated to exhibit therequisite viscoelastic properties resulting in a desired balance oftack, peel adhesion, and shear holding power. Obtaining the properbalance of properties is not a simple process. A quantitativedescription of PSAs can be found in the Dahlquist reference cited above.

Useful PSAs are described in detailed in the Sherman et al. referencescited above. Only a brief description of useful PSAs is included here.Exemplary poly(meth)acrylate PSAs are derived from: monomer A comprisingat least one monoethylenically unsaturated alkyl (meth)acrylate monomerand which contributes to the flexibility and tack of the PSA; andmonomer B comprising at least one monoethylenically unsaturatedfree-radically copolymerizable reinforcing monomer which raises the Tgof the PSA and contributes to the cohesive strength of the PSA. MonomerB has a homopolymer glass transition temperature (Tg) higher than thatof monomer A. As used herein, (meth)acrylic refers to both acrylic andmethacrylic species and likewise for (meth)acrylate.

Preferably, monomer A has a homopolymer Tg of no greater than about 0°C. Preferably, the alkyl group of the (meth)acrylate has an average ofabout 4 to about 20 carbon atoms. Examples of monomer A include2-methylbutyl acrylate, isooctyl acrylate, lauryl acrylate,4-methyl-2-pentyl acrylate, isoamyl acrylate, sec-butyl acrylate,n-butyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, n-octylacrylate, n-decyl acrylate, isodecyl acrylate, isodecyl methacrylate,isononyl acrylate, and mixtures thereof. The alkyl group can compriseethers, alkoxy ethers, ethoxylated or propoxylated methoxy(meth)acrylates. Monomer A may comprise benzyl acrylate.

Preferably, monomer B has a homopolymer Tg of at least about 10° C., forexample, from about 10 to about 50° C. Monomer B may comprise(meth)acrylic acid, (meth)acrylamide and N-monoalkyl or N-dialkylderivatives thereof, or a (meth)acrylate. Examples of monomer B includeN-hydroxyethyl acrylamide, diacetone acrylamide, N,N-dimethylacrylamide, N, N-diethyl acrylamide, N-ethyl-N-aminoethyl acrylamide,N-ethyl-N-hydroxyethyl acrylamide, N,N-dihydroxyethyl acrylamide,t-butyl acrylamide, N,N-dimethylaminoethyl acrylamide, and N-octylacrylamide. Other examples of monomer B include itaconic acid, crotonicacid, maleic acid, fumaric acid, 2,2-(diethoxy)ethyl acrylate,2-hydroxyethyl acrylate or methacrylate, 3-hydroxypropyl acrylate ormethacrylate, methyl methacrylate, isobornyl acrylate, 2-(phenoxy)ethylacrylate or methacrylate, biphenylyl acrylate, t-butylphenyl acrylate,cyclohexyl acrylate, dimethyladamantyl acrylate, 2-naphthyl acrylate,phenyl acrylate, N-vinyl formamide, N-vinyl acetamide, N-vinylpyrrolidone, N-vinyl caprolactam, and mixtures thereof.

In some embodiments, the (meth)acrylate PSA is formulated to have aresultant Tg of less than about 0° C. and more preferably, less thanabout −10° C. Such (meth)acrylate PSAs include about 60 to about 98% byweight of at least one monomer A and about 2 to about 40% by weight ofat least one monomer B, both relative to the total weight of the(meth)acrylate PSA copolymer.

Useful PSAs include natural rubber-based and synthetic rubber-basedPSAs. Rubber-based PSAs include butyl rubber, copolymers of isobutyleneand isoprene, polyisobutylene, homopolymers of isoprene, polybutadiene,and styrene/butadiene rubber. These PSAs may be inherently tacky or theymay require tackifiers. Tackifiers include rosins and hydrocarbonresins.

Useful PSAs include thermoplastic elastomers. These PSAs include styreneblock copolymers with rubbery blocks of polyisoprene, polybutadiene,poly(ethylene/butylene), poly(ethylene-propylene. Resins that associatewith the rubber phase may be used with thermoplastic elastomer PSAs ifthe elastomer itself is not tacky enough. Examples of rubber phaseassociating resins include aliphatic olefin-derived resins, hydrogenatedhydrocarbons, and terpene phenolic resins. Resins that associate withthe thermoplastic phase may be used with thermoplastic elastomer PSAs ifthe elastomer is not stiff enough. Thermoplastic phase associatingresins include polyaromatics, coumarone-indene resins, resins derivedfrom coal tar or petroleum.

Useful PSAs include tackified thermoplastic-epoxy pressure sensitiveadhesives as described in U.S. Pat. No. 7,005,394 (Ylitalo et al.).These PSAs include thermoplastic polymer, tackifier and an epoxycomponent.

Useful PSAs include polyurethane pressure sensitive adhesive asdescribed in U.S. Pat. No. 3,718,712 (Tushaus). These PSAs includecrosslinked polyurethane and a tackifier.

Useful PSAs include polyurethane acrylate as described in US2006/0216523 (Shusuke). These PSAs include urethane acrylate oligomer,plasticizer and an initiator.

Useful PSAs include silicone PSAs such as polydiorganosiloxanes,polydiorganosiloxane polyoxamides and silicone urea block copolymersdescribed in U.S. Pat. No. 5,214,119 (Leir, et al). The silicone PSAsmay be formed from a hyrosilylation reaction between one or morecomponents having silicon-bonded hydrogen and aliphatic unsaturation.The silicone PSAs may include a polymer or gum and an optionaltackifying resin. The tackifying resin may comprise a three-dimensionalsilicate structure that is endcapped with trialkylsiloxy groups.

Useful silicone PSAs may also comprise a polydiorganosiloxanepolyoxamide and an optional tackifier as described in U.S. Pat. No.7,361,474 (Sherman et al.) incorporated herein by reference. Usefultackifiers include silicone tackifying resins as described in U.S. Pat.No. 7,090,922 B2 (Zhou et al.) incorporated herein by reference.

The PSA may be crosslinked to build molecular weight and strength of thePSA. Crosslinking agents may be used to form chemical crosslinks,physical crosslinks or a combination thereof, and they may be activatedby heat, UV radiation and the like.

In some embodiments, the viscoelastic lightguide comprises a PSA formedfrom a (meth)acrylate block copolymer as described in U.S. Pat. No.7,255,920 B2 (Everaerts et al.). In general, these (meth)acrylate blockcopolymers comprise: at least two A block polymeric units that are thereaction product of a first monomer composition comprising an alkylmethacrylate, an aralkyl methacrylate, an aryl methacrylate, or acombination thereof, each A block having a Tg of at least 50° C., themethacrylate block copolymer comprising from 20 to 50 weight percent Ablock; and at least one B block polymeric unit that is the reactionproduct of a second monomer composition comprising an alkyl(meth)acrylate, a heteroalkyl (meth)acrylate, a vinyl ester, or acombination thereof, the B block having a Tg no greater than 20° C., the(meth)acrylate block copolymer comprising from 50 to 80 weight percent Bblock; wherein the A block polymeric units are present as nanodomainshaving an average size less than about 150 nm in a matrix of the B blockpolymeric units.

In some embodiments, the viscoelastic lightguide comprises a clearacrylic PSA, for example, those available as transfer tapes such as VHB™Acrylic Tape 4910F from 3M Company and 3M™ Optically Clear LaminatingAdhesives (8140 and 8180 series).

In some embodiments, the viscoelastic lightguide comprises a PSA formedfrom at least one monomer containing a substituted or an unsubstitutedaromatic moiety as described in U.S. Pat. No. 6,663,978 B1 (Olson etal.).

In some embodiments, the viscoelastic lightguide comprises a copolymeras described in U.S. Pat. Nos. 8,378,046 and 8,772,425 (Determan etal.), comprising (a) monomer units having pendant bephenyl groups and(b) alkyl (meth)acrylate monomer units.

In some embodiments, the viscoelastic lightguide comprises a copolymeras described in U.S. Pat. No. 8,309,650 (Determan et al.), comprising(a) monomer units having pendant carbazole groups and (b) alkyl(meth)acrylate monomer units.

In some embodiments, the viscoelastic lightguide comprises an adhesiveas described in U.S. Publication No. 2010/0297406 (Schaffer et al.),comprising a block copolymer dispersed in an adhesive matrix to form aLewis acid-base pair. The block copolymer comprises an AB blockcopolymer, and the A block phase separates to form microdomains withinthe B block/adhesive matrix. For example, the adhesive matrix maycomprise a copolymer of an alkyl (meth)acrylate and a (meth)acrylatehaving pendant acid functionality, and the block copolymer may comprisea styrene-acrylate copolymer. The microdomains may be large enough toforward scatter incident light, but not so large that they backscatterincident light. Typically these microdomains are larger than thewavelength of visible light (about 400 to about 700 nm). In someembodiments the microdomain size is from about 1.0 to about 10 um.

The viscoelastic lightguide may comprise a stretch releasable PSA.Stretch releasable PSAs are PSAs that can be removed from a substrate ifthey are stretched at or nearly at a zero degree angle. In someembodiments, the viscoelastic lightguide or a stretch release PSA usedin the viscoelastic lightguide has a shear storage modulus of less thanabout 10 MPa when measured at 1 rad/sec and −17° C., or from about 0.03to about 10 MPa when measured at 1 rad/sec and −17° C. Stretchreleasable PSAs may be used if disassembling, reworking, or recycling isdesired.

In some embodiments, the stretch releasable PSA may comprise asilicone-based PSA as described in U.S. Pat. No. 6,569,521 B1 (Sheridanet al.) or U.S. Publication No. 2011/0020640 (Sherman et al.) and U.S.Pat. No. 8,673,419 (Determan et al.). Such silicone-based PSAs includecompositions of an MQ tackifying resin and a silicone polymer. Forexample, the stretch releasable PSA may comprise an MQ tackifying resinand an elastomeric silicone polymer selected from the group consistingof urea-based silicone copolymers, oxamide-based silicone copolymers,amide-based silicone copolymers, urethane-based silicone copolymers, andmixtures thereof.

In some embodiments, the stretch releasable PSA may comprise anacrylate-based PSA as described in U.S. Pat. No. 8,557,378 (Yamanaka etal.) and U.S. Publication No. 2011/0268929 (Tran et al.) Suchacrylate-based PSAs include compositions of an acrylate, an inorganicparticle and a crosslinker. These PSAs can be a single or multilayer.

The viscoelastic lightguide may comprise an aerogel. An aerogel is alow-density solid state material derived from gel in which the liquidcomponent of the gel has been replaced with air. Silica, alumina andcarbon aerogels are exemplary aerogels that may be used.

The viscoelastic lightguide can optionally include one or more additivessuch as filler, particles, plasticizers, chain transfer agents,initiators, antioxidants, stabilizers, fire retardants, viscositymodifying agents, foaming agents, antistats, colorants such as dyes andpigments, fluorescent dyes and pigments, phosphorescent dyes andpigments, fibrous reinforcing agents, and woven and non-woven fabrics.

The viscoelastic lightguide may be made hazy and/or diffusive byincluding particles such as nanoparticles (diameter less than about 1um), microspheres (diameter 1 um or greater), or fibers. Exemplarynanoparticles include TiO₂. Haze and diffusive properties can also beincorporated into the viscoelastic lightguide by incorporating bubblesinto the lightguide. The bubbles may have a diameter of from about 0.01to about 50 um. Bubbles may be introduced by adding, e.g., foamingagents. Examples of additional additives that may be added to theviscoelastic lightguide include glass beads, reflective particles, andconductive particles. In some embodiments, the viscoelastic lightguidemay comprise a PSA matrix and particles as described in U.S. PublicationNo. 2011/0165361 (Sherman et al.), comprising an optically clear PSA andsilicon resin particles having a refractive index less than that of thePSA, and incorporated herein by reference. In some embodiments, thepresence of particles, bubbles, air, etc. increases the scatter anduniformity of light.

Exemplary optical device 100 further comprises retroreflective film 140having layer of transparent microspheres 141 and reflective material 142disposed behind the microspheres. In general, the phrase “behind themicrospheres” is in relation to eye 120. In general, “transparent”refers to being capable of transmitting light. Light propagating withinthe viscoelastic lightguide may be extracted, as shown by ray 131, fromthe lightguide and retroreflected by retroreflective film 140.

The retroreflective film may comprise a binder for supporting thetransparent microspheres. For example, retroreflective film 140 furthercomprises binder 143, wherein the transparent microspheres are partiallyembedded in the binder, and the reflective material is disposed betweenthe microspheres and the binder. In some embodiments, the microspheresmay be completely embedded in a binder. In this case, the binder needsto be transparent so that light can reach the transparent microspheres.FIG. 2 shows a schematic cross section of exemplary optical device 200having a front lit configuration. Retroreflective film 240 compriseslayer of transparent microspheres 241 completely embedded in transparentbinder 243 with reflective material 242 disposed behind the microspheresand between the microspheres and the transparent binder.

In general, a retroreflective film is suitable for retroreflecting lightif light incident upon the film is reflected back in one or more usefuldirections relative to the direction of the incident light. Light may bereflected back in a direction that is 180° or nearly 180° from that ofthe incident light. Light may be reflected back in one or moredirections that are anywhere from about 45° to about 180° relative tothe direction of the incident light.

The retroreflective film may have opposing major surfaces that aresubstantially unstructured, structured with a plurality of features, ora combination thereof. In FIG. 1, the upper surface (adjacent theviscoelastic lightguide) of retroreflective film 140 includes surfacesof the transparent microspheres and thus is structured with a pluralityof lenticular features. This upper surface also comprises the reflectivematerial and the binder which together form a substantially planarsurface between the lenticular features. The lower surface opposite theupper surface is substantially unstructured. In FIG. 2, both upper andlower surfaces of retroretroreflective film 240 are substantiallyunstructured. A surface of the retroreflective film may comprise any oneof the plurality of features described above for the viscoelasticlightguide.

The retroreflective film may comprise retroreflective sheeting sometimesreferred to as beaded retroreflective sheeting. This type of sheeting isemployed in many traffic safety and personal safety articles such asroad signs, barricades, license plates, pavement markers and markingtape, as well as retroreflective tapes for vehicles and clothing.Exemplary retroreflective sheeting is described in Tung et al.; U.S.Pat. No. 2,440,584 (Heltzer et al.); U.S. Pat. No. 5,064,272 (Bailey etal.); U.S. Pat. No. 4,082,426 (Brown); U.S. Pat. No. 4,418,110 (May etal.); US 2007/0110960 (Frey et al.); and U.S. Pat. No. 8,783,879 (Smithet al.); all of which are incorporated herein by reference.

The retroreflective film comprises transparent microspheres. In general,transparent means that a given material is capable of transmittinglight. The transparent microspheres may have a diameter of from about 50to about 200 um, or from about 50 to about 100 um. Light extracted fromthe viscoelastic lightguide enters the transparent microspheres so thatthe light can be retroreflected. Thus, the transparent microspheres aretypically extractor elements. They may have a refractive index greaterthan that of the material(s) from which they extract light, e.g., fromabout 1.4 to about 2.5, or from about 1.8 to about 2.5. The transparentmicrospheres may comprise glass, glass-ceramic, and/or polymer.

The reflective material disposed behind the transparent microspheres isused to facilitate retroreflection of light within the microspheres. Thereflective material may be positioned anywhere behind the transparentmicrospheres as long as the optical device can function as desired.Reflective material may be in front of, or on the side of, thetransparent microspheres as long as the optical device can function asdesired. The reflective material may be disposed as a hemisphericalshell that covers half or nearly half of each microsphere as shown inFIG. 1, or reflective material may be disposed on a small portion behindeach microsphere as shown in FIG. 2. The reflective material may bediscontinuous across the retroreflective film as shown in FIGS. 1 and 2,discontinuous meaning that there are areas between the transparentmicrospheres that do not comprise the reflective material.

The reflective material may be in the form of a layer that may bestructured on one or both sides. FIG. 3 shows a schematic cross sectionof exemplary optical device 300 comprising viscoelastic lightguide 110on top of retroreflective film 340. Optical device 300 further comprisesretroreflective film 340 having layer of transparent microspheres 341and reflective material 342 disposed as a layer behind the microspheres.FIG. 4 shows a schematic cross section of exemplary optical device 400comprising viscoelastic lightguide 110 on top of retroreflective film440. Optical device 400 further comprises retroreflective film 440having layer of transparent microspheres 441 and reflective material 442disposed as a layer behind the microspheres. The optical devices ofFIGS. 3 and 4 comprise lower binder layers 343 and 443, respectively,and FIG. 3 comprises upper binder layer 344. These binder layers aredescribed below.

The reflective material may function as a binder such that it supportsthe transparent microspheres. This reflective binder may be a lowerbinder layer. FIG. 5 shows a schematic cross section of exemplaryoptical device 500 comprising viscoelastic lightguide 110 andretroreflective film 540 having layer of transparent microspheres 541and reflective material 542 disposed as a layer behind the microspheres.The transparent microspheres are partially embedded in the reflectivebinder.

The retroreflective film may further comprise a transparent binder,wherein the transparent microspheres are at least partially embedded inthe transparent binder, and the transparent binder is disposed betweenthe viscoelastic lightguide and the layer of transparent microspheres.FIG. 6 shows a schematic cross section of exemplary optical device 600comprising viscoelastic lightguide 110 and retroreflective film 640having layer of transparent microspheres 641 and reflective material 642disposed as a layer behind the microspheres. Optical device 600comprises upper binder layer 643 which is transparent. In general, theupper transparent binder layer and the reflective material may be indirect contact as is shown in FIG. 6.

The reflective material may be selected to reflect a certain amount oflight that is propagating within the transparent microspheres. Theamount of light reflected by the reflective material may be from about10 to about 50%, from about 50 to about 70%, from about 70 to about100%, or from about 80 to about 100% relative to the total amount oflight that enters the retroreflective layer.

The reflective material may function as a specular reflector wherein thereflection angle of light is within about 16° of the incident angle. Aspecular reflector may be fully or near fully specular as a reflectorover some range of incident angles. Also, specular reflectors may befrom about 85 to about 100% reflective across a particular region of theelectromagnetic spectrum, for example, the visible region.

The reflective material may function as a diffuse reflector whereinlight of a given incident angle reflects with multiple reflection anglessuch that the light is scattered, with at least some of the reflectionangles being greater than about 16° of the incident angle. A diffusereflector may be fully or near fully reflective over some range ofincident angles. Also, diffuse reflectors may be from about 85 to about100% reflective across a particular region of the electromagneticspectrum, for example, the visible region.

The reflective material may comprise a metal such as aluminum or silver.These materials may be vapor coated. The reflective material maycomprise a polymer. The reflective material may comprise polymers suchas acrylates and methacrylates and may have particles for diffusinglight. The reflective material may be a paint or an enamel.

The retroreflective film may comprise one or more polymer layers thatmay function as binders. For example, in FIG. 3, retroreflective film340 comprises upper binder 344 and lower binder 343. In general, “upper”and “lower” are with respect to the layer of transparent microspheresand the position of the viewer as indicated by eye 120. For opticaldevice 300, the binder layers are separated by reflective layer 342. Foroptical device 600, the binder layers are in direct contact with eachother. The retroreflective film may comprise only a lower binder layer;optical devices 400 and 500 have lower binder layers 443 and 542,respectively.

The retroreflective film may comprise only an upper binder layer. Theretroreflective film may comprise a transparent binder, wherein thetransparent microspheres are at least partially embedded in thetransparent binder, and the transparent binder is disposed between theviscoelastic lightguide and the layer of transparent microspheres. FIG.7 shows a schematic cross section of exemplary optical device 700comprising viscoelastic lightguide 110 and retroreflective film 740having layer of transparent microspheres 741 and reflective material 742disposed behind the microspheres. Upper binder layer 743 is disposed infront of the transparent microspheres. In general, the upper transparentbinder layer and the reflective material may not be in direct contact asis shown in FIG. 7.

The upper and lower binder materials may be any material as long as theoptical device can function as desired. The lower binder layer may ormay not be reflective. The upper layer needs to have some transparencyso that light can be retroreflected as desired. The upper and lowerbinder layers may be the same or different.

The transparent microspheres and the reflective material may or may notbe in contact with each other. The retroreflective film may furthercomprise a transparent binder, wherein the transparent microspheres areat least partially embedded in the transparent binder, and thetransparent binder is disposed between the transparent microspheres andthe reflective material. FIG. 8 shows a schematic cross section ofexemplary optical device 800 comprising viscoelastic lightguide 110 andretroreflective film 840 having layer of transparent microspheres 841partially embedded in lower transparent binder layer 843. Reflectivematerial 842 is disposed as a layer on the binder opposite the layer ofmicrospheres. FIG. 9 shows a schematic cross section of exemplaryoptical device 900 comprising viscoelastic lightguide 110 andretroreflective film 940 having layer of transparent microspheres 941partially embedded in lower binder layer 943. Reflective material 942 isdisposed as a structured layer on the binder opposite the layer ofmicrospheres.

The retroreflective film may comprise transparent microspheres that areencapsulated such that a partial interface with air is maintained for atleast one microsphere in the film. FIG. 10 shows a schematic crosssection of exemplary optical device 1000 comprising viscoelasticlightguide 110 and retroreflective film 1040 having layer of transparentmicrospheres 1041 disposed on reflective layer 1042 which is disposed onlower binder layer 1043. The transparent microspheres are surrounded bynetwork of walls 1044 extending from the reflective material to coverfilm 1046 such that at least some of the microspheres are surrounded bythe walls. The network of walls may or may not be transparent. The wallssupport cover film 1046 in spaced relation to the microspheres such thata microsphere/air interface is at least partially maintained for atleast one microsphere in the layer.

Variations of the embodiment shown in FIG. 10 are described in U.S. Pat.No. 4,678,695 (Tung et al.). For example, the transparent microspheresand the cover film may be in contact as shown in FIG. 10, or they maynot be in contact as shown in FIG. 11. FIG. 11 shows a schematic crosssection of exemplary optical device 1100 comprising viscoelasticlightguide 110 disposed on retroreflective film 1140. Theretroreflective film comprises layer of transparent microspheres 1141disposed on reflective layer 1142 which is disposed on lower binderlayer 1143. Groups of the transparent microspheres are surrounded bynetwork of walls 1144 extending from the reflective material to coverfilm 1146. The walls support cover film 1146 in spaced relation to themicrospheres such that a microsphere/air interface is at least partiallymaintained for at least one microsphere in the layer.

The viscoelastic lightguide may be in direct contact with theretroreflective film. FIG. 12 shows a schematic cross section ofexemplary optical device 1200 comprising viscoelastic lightguide 1210and retroreflective film 1240 having layer of transparent microspheres1241 partially embedded in reflective binder layer 1242. Theviscoelastic lightguide and the reflective binder are in direct contactsuch that the transparent microspheres are encapsulated. FIG. 13 shows aschematic cross section of exemplary optical device 1300 comprisingviscoelastic lightguide 110 and retroreflective film 1340 having layerof transparent microspheres 1341 disposed on reflective layer 1342 whichis disposed on lower binder layer 1343. Upper binder layer 1344 isdisposed on the microspheres and the reflective layer such that themicrospheres are encapsulated. Viscoelastic lightguide 110 is disposedon upper binder layer 1344 such that the lightguide and theretroreflective film are in direct contact. FIG. 14 shows yet anothervariation in which the viscoelastic lightguide and the retroreflectivefilm are in direct contact. FIG. 14 shows a schematic cross section ofexemplary optical device 1400 comprising viscoelastic lightguide 110 andretroreflective film 1440 having layer of transparent microspheres 1441at least partially embedded in reflective layer 1442. Upper binder layer1444 is disposed on the microspheres and the reflective layer such thatthe microspheres are encapsulated. Viscoelastic lightguide 110 isdisposed on upper binder layer 1444 such that the lightguide and theretroreflective film are in direct contact.

In general, the retroreflective film is dimensionally stable, durable,weatherable and flexible so that it can be formed into a desired threedimensional shape.

The thickness of the retroreflective film is not particularly limited aslong as it can function as desired. Exemplary thicknesses for theretroreflective film range from about 0.4 mil to about 1000 mil, fromabout 1 mil to about 300 mil, from about 1 mil to about 60 mil, or fromabout 0.5 mil to about 30 mil.

The retroreflective film may have a refractive index of from about 1.45to about 1.65. Materials include plastics such as PLEXIGLAS from Rohmand Haas Co., and polymers of alkylene oxides, vinyl ethers,(meth)acrylates such as polymethylmethacrylate and ethylene/acrylicacids, celluloses, cellulose acetates such as cellulose acetate butyrateand ethylene/vinyl acetates, as well as polyolefins, polyesters,polyurethanes, polycarbonates, epoxies, polyvinylalcohols, natural andsynthetic rubbers, polyacetals, polyacrylonitriles, polycaprolactams,aromatic polysiloxanes, polystyrenes, polyvinylchlorides and nylons. Theretroreflective film may comprise colorants such as particles, dyes orpigments, UV stabilizers and the like.

Given a particular combination of viscoelastic lightguide andretroreflective film, the amount of light retroreflected may be greaterthan about 10%, greater than about 20%, greater than about 30%, greaterthan about 40%, greater than about 50%, greater than about 60%, greaterthan about 70%, greater than about 80%, or greater than about 90%relative to the total amount of light that enters the lightguide. Givena particular combination of viscoelastic lightguide and retroreflectivefilm, the amount of light retroreflected may be from about 10 to about50%, from about 20 to about 50%, from about 30 to about 50%, from about50 to about 70%, from about 50 to about 80%, or from about 10 to about90% relative to the total amount of light that enters the lightguide.

The optical articles of the optical devices described herein can be usedin a variety of multilayer constructions depending on the particularapplication. FIG. 15 shows a schematic cross section of exemplaryoptical device 1500 comprising viscoelastic lightguide 110 andretroreflective film 1540 having layer of transparent microspheres 1541at least partially embedded in a reflective binder 1542. Upper binderlayer 1543 is used to encapsulate the microspheres. First additionallayer 1544 is disposed between the viscoelastic lightguide and upperbinder layer 1543. The first additional layer, being disposed in frontof the transparent microspheres, is transparent such that the opticaldevice can function as desired. Second additional layer 1545 is disposedon viscoelastic lightguide 110 opposite first additional layer 1544. Thesecond additional layer, being disposed in front of the transparentmicrospheres, is transparent such that the optical device can functionas desired. Third additional layer 1546 is disposed on lower binderlayer 1542 opposite layer of transparent microspheres 1541. Any one orcombination of the first, second and third additional layers may beused.

The first optional layer may be designed to interfere or not interferewith the behavior of light being extracted from the viscoelasticlightguide and/or retroreflected by the retroreflective film. The firstoptional layer may have opposing major surfaces that are substantiallyunstructured, structured with a plurality of features, or a combinationthereof.

The thickness of the first optional layer is not limited as long as theoptical device can function as desired. Exemplary thicknesses for thefirst optional layer range from about 0.4 mil to about 1000 mil.

The first optional layer may have a variety of light transmittance andhaze properties. In some embodiments, the first optional layer comprisesan optically clear substrate having high light transmittance of fromabout 80 to about 100%, from about 90 to about 100%, from about 95 toabout 100%, or from about 98 to about 100% over at least a portion ofthe visible light spectrum. In some embodiments, the first optionallayer has low light transmittance, for example, from about 0.1 to about70%, from about 0.1 to about 50%, or from about 0.1 to about 20%. Insome embodiments, the first optional layer has a haze value of fromabout 0.01 to less than about 5%, from about 0.01 to less than about 3%,or from about 0.01 to less than about 1%. In some embodiments, the firstoptional layer is hazy and diffuses light, particularly visible light. Ahazy first optional layer may have a haze value of from about 5 to about90%, from about 5 to about 50%, or from about 20 to about 50%. In someembodiments, the first optional layer is translucent in that it reflectsand transmits light.

In some embodiments, the first optional layer comprises a polymericfilm. Useful polymeric films include cellulose acetate,poly(meth)acrylate (acrylate and/or methacrylate), polyether sulfone,polyurethane, polyester, polycarbonate, polymethyl methacrylate,polyvinyl chloride, syndiotactic polystyrene, cyclic olefin copolymer,polyethylene terephthalate, polyethylene naphthalate, copolymer or blendbased on naphthalene dicarboxylic acids, or some combination thereof. Insome embodiments, the first optional layer comprises apoly(meth)acrylate having a refractive index greater than that of theviscoelastic lightguide.

The first optional layer may comprise glass which generally comprises ahard, brittle, amorphous solid, including, soda-lime glass, borosilicateglass, acrylic glass, sugar glass, and the like.

The first optional layer may provide an image as described below.

The second optional layer may be designed to interfere or not interferewith the behavior of light being extracted from the viscoelasticlightguide and/or retroreflected by the retroreflective film. The secondoptional layer may have opposing major surfaces 1547 and 1548 that aresubstantially unstructured, structured with a plurality of features, ora combination thereof. A surface of the second optional layer maycomprise any one of the plurality of features described above for theviscoelastic lightguide. For example, major surface 1547 may havefeatures comprising lenses which are particularly useful for directinglight to a preferred angular distribution.

The thickness of the second optional layer is not limited as long as theoptical device can function as desired. Exemplary thicknesses for thesecond optional layer range from about 0.4 mil to about 1000 mil.

The second optional layer may have a variety of light transmittance andhaze properties. In some embodiments, the second optional layercomprises an optically clear substrate having high light transmittanceof from about 80 to about 100%, from about 90 to about 100%, from about95 to about 100%, or from about 98 to about 100% over at least a portionof the visible light spectrum. In some embodiments, the second optionallayer has low light transmittance, for example, from about 0.1 to about70%, from about 0.1 to about 50%, or from about 0.1 to about 20%. Insome embodiments, the second optional layer has a haze value of fromabout 0.01 to less than about 5%, from about 0.01 to less than about 3%,or from about 0.01 to less than about 1%. In some embodiments, thesecond optional layer is hazy and diffuses light, particularly visiblelight. A hazy second optional layer may have a haze value of from about5 to about 90%, from about 5 to about 50%, or from about 20 to about50%. In some embodiments, the second optional layer is translucent inthat it reflects and transmits light.

In some embodiments, the second optional layer comprises a polymericfilm. Useful polymeric films include cellulose acetate,poly(meth)acrylate (acrylate and/or methacrylate), polyether sulfone,polyurethane, polyester, polycarbonate, polymethyl methacrylate,polyvinyl chloride, syndiotactic polystyrene, cyclic olefin copolymer,polyethylene terephthalate, polyethylene naphthalate, copolymer or blendbased on naphthalene dicarboxylic acids, or some combination thereof. Insome embodiments, the second optional layer comprises apoly(meth)acrylate having a refractive index greater than that of theviscoelastic lightguide.

The second optional layer may comprise glass which generally comprises ahard, brittle, amorphous solid, including, soda-lime glass, borosilicateglass, acrylic glass, sugar glass, and the like.

The second optional layer may comprise a release liner. Release linerstypically have a low adhesion surface for contact with an adhesivelayer. Release liners may comprise paper such as Kraft paper, orpolymeric films such as poly(vinyl chloride), polyester, polyolefin,cellulose acetate, ethylene vinyl acetate, polyurethane, and the like.The release liner may be coated with a layer of a release agent such asa silicone-containing material or a fluorocarbon-containing material.The release liner may comprise paper or a polymeric film coated withpolyethylene which is coated with a silicone-containing material.Exemplary release liners include liners commercially available from CPFilms Inc. under the trade designations “T-30” and “T-10” that have asilicone release coating on polyethylene terephthalate film.

Exemplary release liners include structured release liners. Exemplaryrelease liners include any of those referred to as microstructuredrelease liners. Microstructured release liners are used to impart amicrostructure on the surface of an adhesive layer. The microstructuredsurface can aid air egress between the adhesive layer and the adjacentlayer. In general, it is desirable that the microstructure disappearover time to prevent interference with optical properties.Microstructures are generally three-dimensional structures that aremicroscopic in at least two dimensions (i.e., the topical and/orcross-sectional view is microscopic). The term “microscopic” as usedherein refers to dimensions that are difficult to resolve by the humaneye without aid of a microscope.

The microstructures may assume a variety of shapes. Representativeexamples include hemispheres, prisms (such as square prisms, rectangularprisms, cylindrical prisms and other similar polygonal features),pyramids, ellipses, grooves (e.g., V-grooves), channels, and the like.In some cases, it may be desirable to include topographical featuresthat promote air egress at the bonding interface when the article islaminated to a substrate. In this regard, V-grooves and channels thatcan extend to the edge of the article are particularly useful. Theparticular dimensions and patterns characterizing the microstructuresare selected based upon the specific application for which the articleis intended. Another example of useful microstructures are described inUS 2007/0292650 A1 (Suzuki) wherein the microstructured adhesive layersurface has one or more grooves that exist only in an inner area of thesurface and are not open at side surfaces of the layer. These groovesmay be in the form of a straight line, branched straight lines, cross,circle, oval, or polygon as viewed from above, and where each form maybe composed of plural discontinuous grooves. These grooves may have awidth of from 5 to 100 micrometers and a depth of from 5 to 50micrometers.

In some embodiments, the first and/or second optional layers may be usedto provide an image. The viscoelastic layer may also provide an image. Avariety of different constructions of the viscoelastic lightguide andthe first and second optional layers may be made to provide an image.The first optional layer may comprise an image printed on either side ofthe layer, or the image may be embedded in the layer. The secondoptional layer may comprise an image printed on either side of thelayer, or the image may be embedded in the layer. ‘The image maycomprise one or more materials different from that of the optionallayer; the one or more materials may be in regions of the layer whereinthe regions are arranged to provide the image. The regions are designedto reflect light or transmit light within a particular range ofwavelengths depending on the particular imaging materials.

Imaging materials may be deposited by printing or marking, e.g., byinkjet printing, laser printing, electrostatic printing and the like.Images may be monochrome such as black and white images, or they may becolored images. Images may comprise one or more colors throughout, e.g.,a uniform layer of color. Images that provide a general or customsurface may be used. For example, an image may be designed such that theoptical article appears as plastic, metal or wood grain; fabric,leather, non-woven, etc. The image may also comprise white dots whichmay be disposed on a surface or interface. The white dots may bearranged as described for extraction features of conventional solidlightguides, e.g., as described in US 2008/232135 A1 (Kinder et al.).Useful imaging materials include those that reflect all or some lightwithin a particular range of wavelengths. Useful imaging materialsinclude those that transmit all or some light within a particular rangeof wavelengths. Exemplary imaging materials include colorants such aspigments and dyes. Imaging materials may also comprise photoniccrystals.

The third optional layer may be a reflector that reflects light beingretroreflected by the retroreflective film. In some embodiments, thereflector comprises a specular reflector wherein the reflection angle oflight is within about 16° of the incident angle. A specular reflectormay be fully or near fully specular as a reflector over some range ofincident angles. Also, specular reflectors may be from about 85 to about100% reflective, from about 90 to about 100%, or from about 95 to about100%, across a particular region of the electromagnetic spectrum, forexample, the visible region. Suitable specular reflectors includemirrors such as a plane mirrors comprising a film of reflectingmaterial, typically a metal, coated on glass.

In some embodiments, the reflector comprises a diffuse reflector whereinlight incident upon the reflector is reflected and scattered at asurface of the diffuse reflector. For a diffuse reflector, light of agiven incident angle reflects with multiple reflection angles wherein atleast some of the reflection angles are greater than about 16° of theincident angle. A diffuse reflector may be fully or near fullyreflective over some range of incident angles. Also, diffuse reflectorsmay be from about 85 to about 100% reflective, from about 90 to about100%, or from about 95 to about 100%, across a particular region of theelectromagnetic spectrum, for example, the visible region. The diffusereflector may comprise a layer of organic, inorganic or hybridorganic/inorganic particles disposed on a substrate. The particles maybe dispersed in a polymeric material or binder. For example, the diffusereflector may comprise a layer of barium sulfate particles loaded in apolyethylene terephalate film.

In some embodiments, the viscoelastic lightguide may comprise a PSAmatrix and particles as describe in U.S. Pat. No. 6,288,172 (Goetz etal.) comprising a mixture of a PSA matrix having a refractive index ofn₁ filled with organic, polymeric microparticles having a refractiveindex n₂ wherein n₁ is greater than n₂. For example, PSA matrix is afilm forming or a PSA of microsphere-based composition and the organicpolymeric microparticles are prepared from fluorinated acrylate monomersor fluorinated methacrylate monomers having refractive less than that ofthe PSA.

The third optional layer may comprise a multilayer optical film havingfrom about 10 to about 10,000 alternating layers of first and secondpolymer layers wherein the polymer layers comprise polyesters. Themultilayer optical film may comprise a three-quarter mirror. Themultilayer optical film may comprise a mirror. The multilayer opticalfilm may comprise a reflective film, a polarizer film, a reflectivepolarizer film, a diffuse blend reflective polarizer film, a diffuserfilm, a brightness enhancing film or a turning film. Exemplarymultilayer optical films are described in U.S. Pat. Nos. 5,825,543;5,828,488 (Ouderkirk et al.); U.S. Pat. Nos. 5,867,316; 5,882,774;6,179,948 B1 (Merrill et al.); U.S. Pat. Nos. 6,352,761 B1; 6,368,699B1; 6,927,900 B2; 6,827,886 (Neavin et al.); U.S. Pat. No. 6,972,813 B1(Toyooka); U.S. Pat. No. 6,991,695; 2006/0084780 A1 (Hebrink et al.);2006/0216524 A1; 2006/0226561 A1 (Merrill et al.); 2007/0047080 A1(Stover et al.); WO 95/17303; WO 95/17691; WO 95/17692; WO 95/17699; WO96/19347; WO 97/01440; WO 99/36248; and WO 99/36262. Exemplary specularreflectors include those available from 3M™ Company, for example, 3M™High Intensity Grade Reflective Products such as High Reflective VisibleMirror Film and High Transmission Mirror Film, and Vikuiti™ films suchas Vikuiti™ Enhanced Specular Reflector.

The third optional layer may comprise aluminum.

The third optional layer may comprise a nanofoam which typicallycomprises a nanostructured, porous material containing pores withdiameters of less than about 100 nm. For example, the third optionallayer may comprise an aerogel which is a low-density solid statematerial derived from gel in which the liquid component of the gel hasbeen replaced with air. Silica, alumina and carbon aerogels areexemplary aerogels that may be used. The third optional layer maycomprise a low refractive index material such as a polymer film filledwith white particles.

The third optional layer may comprise an adhesive layer for attachingthe optical device to some substrate.

The viscoelastic lightguide is adapted to receive at least some lightemitted by the light source. In some embodiments, a specially designedinput surface may not be needed because the light source can be pressedinto the viscoelastic lightguide such that optical coupling occurs. Insome embodiments, the light source may stick to the viscoelasticlightguide, for example, if the lightguide comprises a PSA. In someembodiments, the light source may be embedded in the viscoelasticlightguide.

In some embodiments, the viscoelastic lightguide comprises an inputsurface adapted to receive light from the light source. The inputsurface may have a variety of topographies depending on the opticalcoupling means and/or the particular light source. The input surface mayhave an appropriate curvature. The input edge comprising the inputsurface may have a particular cavity, for example a concavehemispherical cavity, to receive a convex lens of a light source.Alternately, the input surface may have refractive structures such asprisms or lenses to optically couple light from the light source intothe viscoelastic lightguide.

In some embodiments, an extractor article disposed between the lightsource and the input edge may be used to facilitate optical couplingwith at least some of the light emitted by the light source. Usefulextractor articles may have an appropriate curvature for extractinglight from the light source. A coupling material for matching refractiveindices of the viscoelastic lightguide and some element of the lightsource may be used. A crosslinkable material may be used for attachingthe viscoelastic lightguide to some part of the light source, andsubsequently cured using heat and/or light to form the crosslinkedmaterial.

The coupling material may comprise silicone gel. The silicone gel may becrosslinked. The silicone gel may be mixed with a silicone oil. Thesilicone gel may comprise one or more silicone materials such as, forexample, dimethylsilicone, diphenylsilicone, or phenylmethylsilicone.The silicone gel may comprise phenylmethylsilicone moieties that arecross-linked. The silicone gel may comprise phenylmethylsiliconemoieties which are cross-linked and phenylmethylsilicone oil. Thesilicone gel may comprise phenylmethylsilicone moieties which arecross-linked and phenylmethylsilicone oil in a weight ratio from 0.2:1to 5:1. The silicone gel may comprise crosslinked phenylmethylsilicone.Exemplary use of silicone gels is described in U.S. Pat. No. 7,315,418(DiZio et al.) incorporated herein by reference.

The light source may be optically coupled to the viscoelastic lightguidesuch that at least some of the light from the light source can enter thelightguide. For example, a light source may be optically coupled to theviscoelastic lightguide such that from about 1 to about 10%, from about1 to about 20%, from about 1 to about 30%, from about 1 to about 40%,from about 1 to about 50%, from about 1 to about 100%, from about 1 toabout 100%, from about 50 to about 100%, or from about 1 to about 100%of light emitted by the light source enters the viscoelastic lightguide.The light source may emit light having a random or a particular angulardistribution.

The light source may comprise any suitable light source. Exemplary lightsources include linear light sources such as cold cathode fluorescentlamps and point light sources such as light emitting diode (LEDs).Exemplary light sources also include organic light-emitting devices(OLEDs), incandescent bulbs, fluorescent bulbs, halogen lamps, UV bulbs,infrared sources, near-infrared sources, lasers, or chemical lightsources. In general, the light emitted by the light source may bevisible or invisible. At least one light source may be used. Forexample, from 1 to about 10,000 light sources may be used. The lightsource may comprise a row of LEDs positioned at or near an edge of theviscoelastic lightguide. The light source may comprise LEDs arranged ona circuit such that light emitted from the LEDs lights up continuouslyor uniformly the viscoelastic lightguide throughout a desired area. Thelight source may comprise LEDs that emit light of different colors suchthat the colors can mix within the viscoelastic lightguide. In this way,a graphic could be designed to appear differently at different timesduring its use.

The light source may be powered by any suitable means. The light sourcemay be powered using a battery, a DC power supply, an AC to DC powersupply, an AC power supply, or a solar photovoltaic cell.

The viscoelastic lightguide may be made using any method or processcommonly used for making viscoelastic articles. Typical processescomprise those that are continuous processes such as continuous cast andcure, extrusion, microreplication, and embossing methods. Various typesof radiation may be used for processes in which a material needs to becured, e.g., crosslinked. Conventional molding processes may also beused. Molds may be made by micro-machining and polishing of a moldmaterial to create the desired features, structured surfaces, etc. Laserablation may be used to structure a surface of the viscoelasticlightguide and molds. Further detailed description of these processes isdescribed in the Sherman et al. references cited above.

Optical articles comprising the viscoelastic lightguide andretroreflective film may be made in a number of ways. In someembodiments, the lightguide and retroreflective film may be madeseparately, contacted and pressed together using finger pressure, a handroller, an embosser or a laminator. In some embodiments, theretroreflective film may be formed on the viscoelastic lightguide bycoating a retroreflective film material on the lightguide. Theretroreflective film material may then be treated to form theretroreflective film. For example, the retroreflective film material maybe extruded onto the viscoelastic lightguide in the form of a layer andcooled to solidify the material to form the retroreflective film.Alternatively, the retroreflective film material may be curable andtreated by heating and/or applying radiation to form the retroreflectivefilm. The retroreflective film material may include solvent and theretroreflective film is formed by removing the solvent.

In some embodiments, the viscoelastic lightguide may be formed on theretroreflective film by coating a viscoelastic material on theretroreflective film. The viscoelastic material may then be treated toform the viscoelastic lightguide. For example, the viscoelastic materialmay be extruded onto the retroreflective film in the form of a layer andcooled to solidify the material to form the lightguide. Alternatively,the viscoelastic material may be curable and treated by heating and/orapplying radiation to form the lightguide. The viscoelastic material mayinclude solvent and the lightguide is formed by removing the solvent.

In cases where the retroreflective film material or the viscoelasticmaterial is curable, an optical article having a partially curedretroreflective film or lightguide, respectively, may be made. In caseswhere the retroreflective film material or the viscoelastic material iscurable, chemically curing materials may be used such that the materialis crosslinked. In cases where the retroreflective film material or theviscoelastic material is curable, the retroreflective film material maybe cured before, after and/or during contact with another material orthe light source.

In cases where the retroreflective film material or the viscoelasticmaterial is curable using light, the light source may be opticallycoupled to the material and curing carried out by injecting light fromthe light source.

A retroreflective film may be used to structure a surface of theviscoelastic lightguide, e.g., the viscoelastic lightguide may not bestructured by itself, rather, it becomes structured when contacted witha structured surface of a retroreflective film. It is also possible forthe viscoelastic lightguide to have a structured surface such that itdeforms a surface of a retroreflective film to create the interface.

The optical articles and optical devices disclosed herein may beprovided in any number of ways. The optical articles and optical devicesmay be provided as sheets or strips laid flat, or they can be rolled upto form a roll. The optical articles and optical devices may be packagedas single items, or in multiples, in sets, etc. The optical articles andlight sources may be provided in an assembled form, i.e., as an opticaldevice. The optical articles and light sources may be provided as kitswherein the two are separate from each other and assembled at some pointby the user. The optical articles and light sources may also be providedseparately such that they can be mixed and matched according to theneeds of the user. The optical articles and optical devices may betemporarily or permanently assembled to light up.

The optical articles disclosed herein may be altered depending on aparticular use. For example, the optical articles can be cut or dividedby any suitable means, e.g., using a scissors or a die cutting method. Aparticularly useful die cutting method is described in U.S. PublicationNo. 2011/0064916 (Sherman et al.) incorporated herein by reference. Theoptical articles and devices may be cut or divided into different shapessuch as alphabetic letters; numbers; geometric shapes such as squares,rectangles, triangles, stars and the like.

The optical articles and optical devices may be used for signage such asfor graphic arts applications. The optical articles and optical devicesmay be used on or in windows, walls, wallpaper, wall hangings, pictures,posters, billboards, pillars, doors, floormats, vehicles, or anywheresignage is used. Exemplary optical articles a may be used on curvedsurfaces as shown in FIG. 19 of U.S. Publication No. 2011/0176325(Sherman et al.).

The optical articles and optical devices may be double-sided such thatlight can be observed on both sides of a sign, marking, etc. FIG. 16shows a schematic cross section of exemplary optical device 1600comprising first viscoelastic lightguide 1610 a, second viscoelasticlightguide 1610 b, first retroreflective film 1650 a and secondretroreflective film 1650 b. The first and second pairs of viscoelasticlightguide and retroreflective film are disposed on opposite sides ofthird optional substrate 1660. In this embodiment, light is observableon both sides of the device as shown by eyes 1620 a-b.

The optical articles and optical devices may provide images by includingan imaged layer disposed between any two layers described above. FIG. 17shows a schematic cross section of exemplary optical device 1700comprising viscoelastic lightguide 1710 disposed on retroreflective film1740. Retroreflective film 1740 comprises layer of transparentmicrospheres 1741 at least partially embedded in lower binder layer 1742with upper binder layer 1750 encapsulating the microspheres. Thirdoptional layer 1743 is disposed on the retroreflective film opposite theviscoelastic lightguide. Imaging layer 1760 is disposed between thelower binder layer and the third optional layer. The imaging layer isshown as being discontinuous, but it may also be a continuous layer suchas a film or sheet disposed between the two other layers. The imaginglayer may be made by printing as described above.

The optical articles and devices may be used for safety purposeswherever light is desired. For example, the optical articles and devicesmay be used to illuminate one or more steps of a ladder, steps of astairway, aisles such as in airplanes and movie theatres, walkways,egress, handrails, work zone identification signs and markings. Anexemplary optical article for one of these applications is shown in FIG.20 of U.S. Publication No. 2011/0176325 (Sherman et al.).

The optical articles and optical devices may be used in a variety ofitems such as reading lights; party and holiday decorations such ashats, ornaments, string lighting, balloons, gift bags, greeting cards,wrapping paper; desk and computer accessories such as desk mats,mousepads, notepad holders, writing instruments; sporting items such asfishing lures; craft items such as knitting needles; personal items suchas toothbrushes; household and office items such as clock faces, wallplates for light switches, hooks, tools. An exemplary optical articlefor one of these applications is shown in FIG. 21 of U.S. PublicationNo. 2011/0176325 (Sherman et al.).

The optical articles and optical devices may be used on clothing andclothing accessories for decorative and/or safety purposes. For example,the optical articles and optical devices may be used on outerwear forcyclists, or on clothing or headgear for miners. For another example,the optical articles and optical devices may be used on or in straps andwristbands of watches, or on or in watch faces. An exemplary opticalarticle for one of these applications is shown in FIG. 22 of U.S.Publication No. 2011/0176325 (Sherman et al.).

The optical articles and optical devices may be used anywhere light isneeded or desired. The optical articles and optical devices may bedisposed on a top surface of a shelf such that light from the article ordevice, respectively, is emitted in an upward direction. Likewise, theoptical articles and optical devices may be disposed on a bottom surfaceof a shelf such that light from the article or device, respectively, isemitted in a downward direction. The optical articles and opticaldevices may also be disposed on or within a shelf having a lighttransmissive portion. The articles and devices may be arranged such thatlight is emitted from the light transmissive portion. An exemplaryoptical article for one of these applications is shown in FIG. 23 ofU.S. Publication No. 2011/0176325 (Sherman et al.).

The optical articles and devices may be used as flashlights. Forexample, optical articles and optical devices may be disposed on theoutside or inside of a battery cover plate or other part of anelectronic handheld device. The optical articles and optical devices mayor may not be hardwired to the electronic device's battery but couldhave its own power source. The electronic device's battery cover may ormay not be removable from the rest of the device comprising the display.

The optical articles and optical devices may be used for vehicles suchas automobiles, marine vehicles, buses, trucks, railcars, trailers,aircraft, and aerospace vehicles. The optical articles and devices maybe used on almost any surface of a vehicle including the exterior,interior, or any in-between surface. For example, the optical articlesand devices may be used to light up door handles on the exterior and/orinterior of a vehicle. The optical articles and devices may be used toilluminate trunk compartments, for example, they may be positioned onthe underside of the trunk lid or inside the compartment. The opticalarticles and devices may be used on bumpers, spoilers, floor boards,windows, on or as tail lights, sill plate lights, puddle lights,emergency flashers, center high mounted stop lights, or side lights andmarkers. The optical articles and devices may be used to illuminate theinside of engine compartments, for example, they may be positioned onthe underside of the hood, inside the compartment, or on an engine part.

The optical articles and devices may also be used on the edge surfacesof vehicular doors between the exterior and interior panels of thedoors. These optical articles and devices may be used to provide avariety of information for the user, manufacturer, etc. The opticalarticles and devices may be used to illuminate the instrument panel of avehicle where lighted areas are typically displayed. The opticalarticles and devices may be used on other interior items such ascupholders, consoles, handles, seats, doors, dashboards, headrests,steering wheels, wheels, portable lights, compasses, and the like. Theoptical articles and devices may be used on back or pass areas forreading light or to provide ambient lighting for inside a vehicle. FIG.24 U.S. Publication No. 2011/0176325 (Sherman et al.) shows an exemplaryautomobile having optical articles 2400 and 2401.

The optical articles and optical devices may be used in the manufactureof an item or as a replacement part of an item. For example, the opticalarticles and optical devices may be sold to an automobile manufactureror automobile repair shop for assembly or repair of some specific partof an automobile. FIG. 25 of U.S. Publication No. 2011/0176325 (Shermanet al.) shows an exemplary automobile having tail light 2500. An opticalarticle or optical device (not shown) is disposed behind the outsidelayer of the tail light which is typically red, yellow or clear plastic.The tail light may comprise a cavity with a light bulb or LED as a lightsource. An optical article or device may be used in the cavity as areplacement for the light source. Alternatively, the tail light may notcomprise a cavity or at least comprise a much smaller cavity than isused in today's automobiles. An optical article or optical device may bedisposed behind or within the outside layer of the tail light such thatthe overall size of the tail light is reduced.

The optical articles and optical devices may be used for traffic safetysuch as for traffic signs, street signs, highway dividers and barriers,toll booths, pavement markings, and work zone identification signs andmarkings. The optical articles and devices may be used on license platesfor decoration, to provide information such as vehicle registration,etc. The optical articles and devices may also be used to provide lightnear license plates such that the license plates are lit up from theside, top, etc. The optical articles and optical devices may be used inthe construction of a front lit illuminated license plate assembly.

The optical articles and optical devices may be used on or in displaydevices such as cell phones, personal digital devices, MP3 players,digital picture frames, monitors, laptop computers, projectors such asmini-projectors, global positioning displays, televisions, etc. Thedisplay device can be assembled without the need for adhesives to bonddisplay components to the viscoelastic lightguide. The optical articlesand devices may also be used for lighting buttons and keypads in variouselectronic devices.

In some embodiments, particularly in optical devices having a front litconfiguration or optical articles designed to be used in a front litconfiguration, the retroreflective film suitable for retroreflectinglight may comprise a layer of transparent microspheres without anyreflective material disposed behind the microspheres. FIG. 18 shows aschematic cross section of exemplary optical device 1800 having a frontlit configuration. Viscoelastic lightguide 110 is on top ofretroreflective film 1840 comprising layer of transparent microspheres1841. The transparent microspheres may be at least partially embedded intransparent binder 1843. The reflective material is not needed if theretroreflective film can retroreflect light sufficiently so that theoptical device can function as desired. For example, the refractiveindex difference between the transparent microspheres and transparentbinder may be large enough so that sufficient light is retroreflectedand not refracted at the interface between the microspheres and binder.

In some embodiments, the retroreflective film suitable forretroreflecting light may provide a virtual or floating image in whichan image is formed and appears to float above or below the layer oftransparent microspheres, or both. FIG. 19 shows a schematic crosssection of exemplary optical device 1900 having a front litconfiguration. Viscoelastic lightguide 110 is on top of retroreflectivefilm 1940 comprising layer of transparent microspheres 1941. Thetransparent microspheres may be at least partially embedded in binder1943 which may or may not be transparent. Reflective material 1942 isassociated with each transparent microsphere. The reflective materialforms an individual image, and a composite image is formed by theindividual images such that the composite image appears to the unaidedeye to float in front of the optical article as indicated by eye 120.

Reflective material 1942 typically comprises a radiation sensitivematerial which, upon exposure to radiation, changes to provide acontrast with reflective material that was not exposed to the radiation.Exemplary radiation sensitive materials include metals such as aluminum,silver, copper and gold; metal oxides such as aluminum oxide; andnon-metallic materials such as zinc sulfide and silicon dioxide.Exemplary radiation sources include any of those that emit light in awavelength range of from about 200 nm to about 11 um. The reflectivematerial may be imaged by directing collimated light through a lenstoward the front side of the retroreflective film. The image may beformed by inducing a compositional or color change in the reflectivematerial, or by removal of the material.

In some embodiments, an optical device having a back lit configurationmay provide a floating image. FIG. 20 shows a schematic cross section ofexemplary optical device 2000 in which viscoelastic lightguide 110 isbelow retroreflective film 140 or farther than the retroreflective filmto the viewer as indicated by eye 120.

In some embodiments, retroreflective film used to provide floatingimages may comprise a layer of transparent microspheres without anyreflective material disposed on the microspheres. The microspheres areimaged using radiation.

In some embodiments, floating images having more than one color may bemade using an optical device having both a front and back litconfiguration. FIG. 21 shows a schematic cross section of exemplaryoptical device 2100. Retroreflective film 2130 comprises layer oftransparent microspheres 2131 embedded in transparent polymeric binder2133, and reflective material 2132 is associated with each microsphere.On opposing sides of retroreflective film 2130 are disposed lowerviscoelastic lightguide 2110 and upper viscoelastic lightguide 2115.Optional reflective layer 2120 is disposed on lower viscoelasticlightguide 2110 opposite the retroreflective film. Lower light source2105 emits light represented by rays 2106 having a first color. Lowerlight source 2105 is positioned relative to lower viscoelasticlightguide 2110 such that light emitted by light source 2105 entersviscoelastic lightguide 2110 and is transported within the lightguide bytotal internal reflection. Upper light source 2107 emits lightrepresented by rays 2108 having a second color. Upper light source 2107is positioned relative to upper viscoelastic lightguide 2115 such thatlight emitted by light source 2107 enters viscoelastic lightguide 2115and is transported within the lightguide by total internal reflection.The first and second colors may be different, or they may be the same.

Detailed description of materials and methods used to form floatingimages are described in U.S. Pat. No. 6,288,842 B1 (Florczak et al.);U.S. Pat. No. 7,336,422 (Dunn et al.); US 2008/0130126 A1 (Brooks etal.); US 2007/0081254 A1 (Endle et al.); US 2008/0118862 A1; and U.S.Pat. No. 7,995,278 (Endle et al.); all of which are incorporated byreference.

Example

An adhesive comprising 85/14/1 by weight of isooctyl acrylate/isobornylacrylate/acrylic acid, 0.08 wt. % 1-6-hexanediol diacrylate and 0.20 wt.% IRGACURE 651 (Ciba Specialty) was coated onto a 2.5 mil thickpolymeric mirror film (Vikuiti™ Enhanced Specular Reflector from 3M™Co.), using a notched bar knife coater. The adhesive was coated with awet thickness of 50 mils (1250 um) on one side and 26 mils (1000 um) onthe other to give a slight wedge. The adhesive coating was covered witha silicone release liner (CP Films T10 2.0 mil polyester release liner)and cured using a low intensity UV lamp for 15 minutes. The adhesive hada refractive index of 1.474 as measured on an Abbe refractometer. Afterthe curing was completed, the release liner was removed and a virtualimage graphic film was then laminated to the adhesive layer surfaceopposite the polymeric mirror film (4 inch by 3.25 inch area). A 4 inchlong side-emitting LED circuit (Honglitronic Part No.HL-4008U4R30K16N-12 flexible strip with red LEDs) was pressed into thecured PSA layer and light was easily passed through the entire 3.5inches of PSA length and was able to be visibly seen exiting through theends.

On top of the virtual image graphic film another adhesive wedge filmcured between two release liner sheets, was laminated on top. A 4 inchlong side-emitting LED circuit (Honglitronic Part No. HL-LR4008W-SESMflexible strip with white LEDs) was pressed into the adhesive layer ontop of the virtual image film. The adhesive face left covered with (CPFilms T10 2.0 mil polyester release liner). The LEDs were powered with a12 volt source with less than 1 amp current. The red light was seen topass from the bottom lightguide layer in the image shape, and the whitelight was seen to bounce from the top surface of the reflective sheetingto produce white light illumination.

The terms “in contact” and “disposed on” are used generally to describethat two items are adjacent one another such that the whole item canfunction as desired. This may mean that additional materials can bepresent between the adjacent items, as long as the item can function asdesired.

What is claimed is:
 1. An optical device comprising: a light source; aviscoelastic lightguide, wherein light emitted by the light sourceenters the viscoelastic lightguide and is transported within thelightguide by total internal reflection; and a retroreflective filmsuitable for retroreflecting light and comprising a layer of transparentmicrospheres and reflective material disposed in back of the layer;wherein the viscoelastic lightguide is disposed in front of the layer oftransparent microspheres such that light being transported within theviscoelastic lightguide is extracted from the lightguide andretroreflected by the transparent microspheres.
 2. The optical device ofclaim 1, the retroreflective film further comprising a binder, whereinthe transparent microspheres are partially embedded in the binder, andthe reflective material is disposed between the microspheres and thebinder.
 3. The optical device of claim 1, the retroreflective filmfurther comprising a transparent binder, wherein the transparentmicrospheres are completely embedded in the transparent binder, and thereflective material is disposed between the microspheres and thetransparent binder.
 4. The optical device of claim 1, wherein thereflective material is discontinuous across the retroreflective film. 5.The optical device of claim 1, wherein the reflective material forms areflective layer.
 6. The optical device of claim 1, wherein thereflective material is a reflective binder, and the transparentmicrospheres are partially embedded in the reflective binder.
 7. Theoptical device of claim 1, the retroreflective film further comprising atransparent binder, wherein the transparent microspheres are at leastpartially embedded in the transparent binder, and the transparent binderis disposed between the viscoelastic lightguide and the layer oftransparent microspheres.
 8. The optical device of claim 1, theretroreflective film further comprising a transparent binder, whereinthe transparent microspheres are at least partially embedded in thetransparent binder, the transparent binder is disposed between theviscoelastic lightguide and the layer of transparent microspheres, andthe transparent binder and the reflective material are in direct contactwith each other.
 9. The optical device of claim 1, the retroreflectivefilm further comprising a transparent binder, wherein the transparentmicrospheres are at least partially embedded in the transparent binder,the transparent binder is disposed between the viscoelastic lightguideand the layer of transparent microspheres, and the transparent binderand the reflective material are not in direct contact with each other.10. The optical device of claim 1, the retroreflective film furthercomprising a transparent binder, wherein the transparent microspheresare at least partially embedded in the transparent binder, and thetransparent binder is disposed between the transparent microspheres andthe reflective material.
 11. The optical device of claim 1, theretroreflective film further comprising a transparent binder, whereinthe transparent microspheres are at least partially embedded in thetransparent binder, the transparent binder is disposed between thetransparent microspheres and the reflective material, and the reflectivematerial forms a reflective layer.
 12. The optical device of claim 1,the retroreflective film further comprising a cover film disposed infront of the transparent microspheres and supported by a network ofwalls extending from the reflective material to the cover film such thatat least some of the transparent microspheres are surrounded by thewalls.
 13. The optical device of claim 1, the retroreflective filmfurther comprising a cover film disposed in front of the transparentmicrospheres and supported by a network of walls extending from thereflective material to the cover film, wherein at least some of thetransparent microspheres are grouped together by the walls.
 14. Theoptical device of claim 1, wherein the viscoelastic lightguide is indirect contact with the retroreflective film.
 15. The optical device ofclaim 1, further comprising one or more optional layers, the optionallayers comprising: a first optional layer disposed between theviscoelastic lightguide and the retroreflective film, a second optionallayer disposed on the viscoelastic lightguide opposite theretroreflective film, and a third optional layer disposed on theretroreflective film opposite the viscoelastic lightguide.
 16. Theoptical device of claim 1, wherein an image is provided by one or moreof: the viscoelastic lightguide, a first optional layer disposed betweenthe viscoelastic lightguide and the retroreflective film, and a secondoptional layer disposed on the viscoelastic lightguide opposite theretroreflective film.
 17. The optical device of claim 1, wherein animage is provided by one or more of: an imaging layer disposed betweenthe layer of transparent microspheres and the reflective material, animaging layer disposed between the retroflective film and theviscoelastic lightguide, and an imaging layer disposed on theviscoelastic lightguide opposite the retroreflective film.
 18. Anoptical device comprising: a light source; a viscoelastic lightguide,wherein light emitted by the light source enters the viscoelasticlightguide and is transported within the lightguide by total internalreflection; and a retroreflective film suitable for retroreflectinglight and comprising a layer of transparent microspheres and notcomprising reflective material disposed in back of the layer; whereinthe viscoelastic lightguide is disposed in front of the layer oftransparent microspheres such that light being transported within theviscoelastic lightguide is extracted from the lightguide andretroreflected by the transparent microspheres.
 19. An optical devicecomprising: a light source and an optical device, the optical devicecomprising: a viscoelastic lightguide, wherein light emitted by thelight source enters the viscoelastic lightguide and is transportedwithin the lightguide by total internal reflection; and aretroreflective film suitable for retroreflecting light and comprising alayer of transparent microspheres, and reflective material associatedwith each transparent microsphere and disposed in back of eachtransparent microsphere; wherein the viscoelastic lightguide is disposedin front of the layer of transparent microspheres such that light beingtransported within the viscoelastic lightguide is extracted from thelightguide and retroreflected by the transparent microspheres, and thereflective material associated with each transparent microsphere formsan individual image, and a composite image is formed by the individualimages such that the composite image appears to the unaided eye to floatin front of the layer of transparent microspheres.
 20. An optical devicecomprising: a light source and an optical device, the optical devicecomprising: a viscoelastic lightguide, wherein light emitted by thelight source enters the viscoelastic lightguide and is transportedwithin the lightguide by total internal reflection; and a reflectivefilm suitable for reflecting light and comprising a layer of transparentmicrospheres, and reflective material associated with each transparentmicrosphere and disposed in back of each transparent microsphere;wherein the viscoelastic lightguide is disposed in back of the layer oftransparent microspheres such that light being transported within theviscoelastic lightguide is extracted from the lightguide and transmittedthrough each of the transparent microspheres, and the reflectivematerial associated with each transparent microsphere forms anindividual image, and a composite image is formed by the individualimages such that the composite image appears to the unaided eye to floatin back of the optical device.